DESCRIPTION: The preliminary data is extensive. It demonstrates that the mode of signalling and the consequences of receptor ligation via the three Fc receptors (FcgammaRI, II, and III and the receptors' A,B,C isoforms) differs with respect to the roles of Ca++, tyrosine phosphorylation, the cytoplasmic tail of the receptor and its interaction with gamma and delta chains. Using transient transfections of COS cells with wild type and mutagenized Fc receptor constructs the Applicant shows tyrosine phosphorylation of either the cytoplasmic tail of FcgammaRII or the gamma or chain are required for complete ligation and phagocytosis via the Fc receptors. Further the transmembrane and the extracellular domains of the molecule, although they regulate the affinity of ligand for receptor, do not materially affect signalling and the functional consequences. The tyrosine phosphorylation occurs via activation and binding of any of several src- related protein tyrosine kinases (PTK) and the Applicant demonstrated the binding and activation, most consistently in phagocytes, of the Syk PTK to the gamma chain (with the FcgammaRI and FcgammaRIII) and the cytoplasmic tail of the FcgRII. The importance of syk is demonstrated by ability of syk to stimulate FCRI phagocytosis via cotransfection with gamma chain in COS. Further, it is clear the exact number and placement of the motif Y-Xn-L in the cytoplasmic tail of the FcgammaRII and all other receptors, except the FcgammaRI where it is lacking, is critical for the complete phagocytic signal. Further it is clear in co- transfectants of COS with FcgammaRI and gamma chain that while the FcRI is alone competent to bind IgG and signal via elevation of cytosolic Ca++, this signal is insufficient to allow phagocytosis; however, cotransfection with gamma chain results in gamma chain phosphorylation and competence of the FcgammaRI for phagocytosis. In specific the Applicant constructed gain of function substitutions which by adding the appropriate number and length of the Y containing motif, a phagocytosis competent FcgammaRII was expressed, which alone +/- the gamma chain could transmit complete phagocytosis signal. Additional preliminary studies show the importance of this Fc receptors in health and disease the modulation of the receptor by cytokines, some evidence that the PKC and Ca++ mediated signal via the FcgammaRI is important for secretion.